The power radiated by an antenna generally varies with direction. A uniformly illuminated aperture has a radiation pattern having a power which varies with respect to angle according to the function (sin x/x).sup.2 as shown in FIG. 1. The angle between half peak power points of a radiation pattern is usually defined as the angular beam-width which is shown as .theta.. Two identical targets are said to be resolved in angle if they are separated by more than the angular beam-width. It is difficult to determine the angular location of a target to an accuracy significantly better than the angular beam-width of an antenna. This is especially the case when the signal-to-noise ratio is low.
Tracking of targets which move with angular velocities requires determination of angular location having an angular accuracy greater than can be achieved using a straightforward single antenna system. Accuracy can be improved by using the technique of spatial interferometry which relies upon having more than one receiving antenna.
In this technique, signals which are received from an object or target are collected by a number of antennae and signals from each antenna are combined by using a comparator to produce a sum channel and a difference channel. The comparator may be a hybrid or an optical circuit. The sum channel produces an output which is the combination of the signals from the antennas. The difference channel produces an error voltage (having a plus or minus sign) which is approximately proportional to the angular deviation of the target from a notional centre line which is referred to as the bore-sight. The bore-sight is the electrical axis of the antenna beam which produces a null out of the difference channel. The sign of the error voltage is determined using a phase-sensitive detector and is used to determine the direction of the angular deviation from the bore-sight.
Spatial interferometry is used in astronomy and radar systems to improve resolution, track moving objects and determine range.
One particular application of spatial interferometry is a monopulse radar system. The term monopulse refers to a radar system which can obtain angular and range information from a single pulse. Such a system has an antenna which can have any number of antenna feeds, but four are commonly used which are placed at the focus of a cassegrainian or a lens system for the reception of signals from a target, normally a moving target A side view of such a system having an antenna (aperture) 10 and an array of horns 12 is shown in FIG. 2. The bore-sight is shown as broken line b. The radar system maintains the position of the target on bore-sight by using information provided by the difference channel to control servo-motors which move the antenna to maintain the target on bore-sight.
When a lens or reflector acts as a receiving antenna the echoes received from the target are not focussed to a point because of the wave nature of the radiation. The radiation is distributed in a diffraction pattern known as an Airy function, the exact nature of the function depending on the energy distribution in the aperture.
If an array of horns is placed in a common plane in the focal region of an antenna, the coupling of energy from free space into the cluster of horns is inefficient. This is because energy distribution in each horn is such that it is a maximum in the centre of the horn and decays towards the walls. Even when the horns are located close together the inefficiency due to coupling loss is large. An array of four horns illuminated by a single aperture has an intrinsic loss of several dB.
The array of horns 12 receives energy reflected or originating from a target. The energy distribution is maximum at the centre of the array of horns where the walls of adjacent horns meet and so coupling is inefficient.